The underdog planet, smaller than the moon, was kicked out of the planetary club by the International Astronomical Union. The 2,500 scientists of the union decided on a definition of a planet as a body that orbits the sun, is so large that its own gravity makes it roughly spherical, and, crucially, also dominates its region of the solar system.

Their decision will force a rewrite of science textbooks because the solar system is now a place with eight planets and three newly defined "dwarf planets" - a new category of object that includes Pluto. The category also includes an asteroid called Ceres and an object bigger than Pluto, initially called 2003 UB313 but later named officially as Eris.